Mask feathering creates hard line?

I have a shot where I have to merge two backgrounds and try to do it seamlessly. It works to an extent when I mask one part over the other, and just feather it out because they're a bit alike. But no matter how much I feather, there is always this obvious hard line in the feather that moves unnaturally.

Is there a way how I can just create a soft transition that isn't too noticeable?

I moved the mask to make it more noticeable. But it's really only noticeable when it's in animation. I just crank up the feathering. But there's always a hard line present. It's close to the center in the first picture.